Dynamically reconfigurable framework for a large-scale battery system

ABSTRACT

A dynamically reconfigurable battery framework for management of a large-scale battery system systems is provided. The framework monitors, reconfigures, and controls large-scale battery systems online. The framework is built upon a topology-based bypassing mechanism that provides a set of rules for changing the battery-pack configuration, and a semantic bypassing mechanism by which the battery-cell connectivity is reconfigured to recover from a battery-cell failure. More specifically, the semantic bypassing mechanism implements a constant-voltage-keeping policy and a dynamic-voltage-allowing policy. The former policy is effective in preventing unavoidable voltage drops during the battery lifetime, while the latter policy is effective in supplying different amounts of power to meet a wide-range of application requirements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S patent applicationSer. No. 12/757,293, filed Apr. 9, 2010 which claims the benefit of U.S.Provisional Application No. 61/168,472 filed on Apr. 10, 2009. Theentire disclosure of the above application is incorporated herein byreference.

FIELD

The present disclosure relates to battery management and, moreparticularly, to a dynamically reconfigurable framework for alarge-scale battery system.

BACKGROUND

Demand for electric vehicles with hybrid drive has soared worldwide duemainly to a recent sharp increase in fuel prices. As oil reservescontinue to dwindle and oil prices rise, and with no viable alternativefuel technologies becoming apparent, the demand for electric vehicleswith hybrid drive will only increase. According to a recent survey, in2008 alone, 36.0% motorists worldwide want to buy a car with hybriddrive while 45.8% are interested in buying full-electric cars. Electriccars are powered entirely with electrical energy from tens of thousandsof battery cells. These battery cells are grouped and assembled as a setof battery packs. Individual cells in a pack, which are exposed to, andmust operate in a harsh environment, have different operatingcharacteristics due to difference in their manufacturing tolerances,uneven temperature conditions across the pack, or non-uniform ageingpatterns. These varied settings, in turn, have crucial effects on thecharge/discharge of battery cells. In a series chain of battery cells, aweak battery cell with low capacity reaches its full charge state wellbefore the rest of the battery cells in the chain, hence overchargingand overheating itself. On the other hand, when the weak cell cannotreach its full charge owing to a high self-discharge and/or ashort-circuited cell, good battery cells may overcharge. In a serieschain of battery cells, an open-circuited cell causes the others in thechain to become open-circuited as well. All of these phenomenaeventually lead to a battery-cell failure, which is inevitableespecially in large-scale battery packs.

The most commonly-used method for managing a large-scale battery systemis module-based, where battery cells are grouped into smaller modules ofbattery cells, each of which is monitored, controlled, and balanced bythe corresponding local controller while a group of modules are managedby a global controller. In such a modular battery-management system,individual electronic control units (ECUs) collect information-such ascell voltage and current, temperature, etc. on their serially-connectedbattery packs via an equalizer connected to each battery cell, and thenprocess and report the collected information to the central ECUresponsible for making the local ECUs work as required.

Individual battery cells can be charged and discharged separately viathe switches around them. Separate discharge or bypassing specificcells, however, requires fine-grained management of battery cellarrangement and battery dynamics while considering their attributes(e.g., cell voltage balancing and capacity efficiency).

Battery-cell failures are inevitable, especially for large-scalebatteries, and the failure rate for a multi-cell battery pack is muchhigher than that of each cell because of inter-cell interactions anddependencies. Unlike the battery packs used for portable electronicdevices, the electronic vehicle environment imposes many challengingrequirements on battery cells and their management.

There are two main challenges in developing a dynamic reconfigurationframework for large-scale battery-management systems. First, theframework should be able to reconfigure battery connectivity online,upon detection of a battery-cell failure. Healthy battery cells shouldalso be kept in use, possibly in the form of two hierarchical layers ofconnectivity: battery cells in each pack (cell-level) and packs in theentire battery system (pack-level). Second, unlike battery-poweredportable devices, a large-scale battery-management system, especiallyfor electric vehicles, requires multiple output terminals of the powersource (from the battery packs), supplying different voltages fordifferent applications and/or devices. Physical separation of batterypacks is, however, rarely an option mainly for cost reasons.

This section provides background information related to the presentdisclosure which is not necessarily prior art.

SUMMARY

A reconfigurable battery system is provided with a plurality of batterycircuits adjoined to each other. The battery circuits include: an inputterminal; an output terminal; a battery cell with a positive terminaland a negative terminal interposed between the input terminal and theoutput terminal; and a plurality of switches interconnecting the batterycell with a battery cell in an adjacent circuit. The plurality of switchmay be configured to place the battery cell in series with the batterycell in the adjacent battery cell, place the battery cell in parallelwith the battery cell in the adjacent battery circuit, or disconnect thebattery cell from the battery cell in the adjacent circuit. A controlunit receives an output criteria and controls the switches in each ofthe battery circuits to form a circuit arrangement that meets the outputcriteria while substantially maximizing power delivered by the circuitarrangement, where the output criteria defines a number of outputs forthe circuit arrangement and a voltage requirement for each output.

The plurality of switches include an input switch connected between theinput terminal and the negative terminal of the battery cell; a parallelswitch connected between the output terminal and the positive terminalof the battery cell; a bypass switch connected between the negativeterminal of the battery cell and a negative terminal of an adjacentbattery circuit; and a series switch connected between the positiveterminal of the battery cell and the negative terminal of the adjacentbattery circuit. The plurality of switches may further include aninput-terminal switch interposed between the input terminal and an inputterminal of the adjacent battery circuit as well as an output-terminalswitch interposed between the output terminal and an output terminal ofthe adjacent battery circuit.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

FIG. 1 is a diagram depicting an exemplary arrangement for areconfigurable battery system;

FIG. 2 is a diagram illustrating reconfiguration of battery cells undera constant-voltage policy;

FIG. 3 is a diagram illustrating reconfiguration of battery cells undera dynamic-voltage allowing policy;

FIG. 4 is a graph illustrating the difference between a reconfigurationscheme and a legacy scheme as it relates to the lifetime of a battery;

FIG. 5 is a graph illustrating battery lifetime gain achieved byreconfiguration as a function of the number of battery cells in a serieschain;

FIG. 6 is a graph illustrating changes in demand voltage andpresentation of the corresponding power;

FIG. 7 is a graph showing a comparison of dynamic voltage allowing andconstant voltage keeping policies with maximum deliverable power; and

FIGS. 8A-8D are graphs depicting dynamic reconfiguration subject to avoltage demand with respect to different discharge rates.

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure. Correspondingreference numerals indicate corresponding parts throughout the severalviews of the drawings.

DETAILED DESCRIPTION

A rechargeable battery cell may be any cell capable of convertingchemical energy to electrical energy, and vice-versa. This is typicallyaccomplished by electrochemical oxidation and reduction reactions. Thesereactions involve the exchange of electrons through the load betweenelectro-active species in two electrodes inside the battery cell,generating a flow of electric current. Ideally, the total number ofcurrent units, or Coulomb, from a battery cell will always be the samethroughout its entire life cycle. In reality, however, thecharacteristics of a battery cell are nowhere close to being ideal dueto the uncertainty of reaction kinetics and diffusion processes and/oractive material dissolution in the battery cell over time. Exemplarybattery cells may include nickel metal hydride (NiMH), lithium ion,nickel cadmium (NiCd), lithium iron phosphate, lithium sulfur,lithium-titanate, nickel hydrogen, nickel-metal hydride, nickel-iron,sodium sulfur, vanadium redox, and rechargeable alkaline. Thearchitecture described below may be applied to these as well as othertypes of rechargeable battery cells.

Rechargeable battery cells exhibit different characteristics inpractice. For instance, the battery terminal voltage is not constantduring its discharge; voltage drops non-linearly with a discharge rate.The higher the discharge rate, the steeper the voltage drop. For thisreason, a DC-DC converter may be used to shift and stabilize the supplyvoltage. Second, battery capacity varies with the discharge rate; thehigher the discharge rate, the lower the battery capacity. Third,batteries have limited charge recovery effects at a high discharge rate.A high load current for a short period of time causes a higherconcentration gradient among electro-active species, making the unusedcharge unavailable due to the lag between reaction and diffusion rates.Thus, when the battery is allowed to rest for some time at a low (orzero) charge rate, the voltage that dropped temporarily goes back up.Last, temperature also affects internal resistance and full chargecapacity. The lower the temperature, the higher the internal resistance,thereby reducing full charge capacity. On the other hand, hightemperature leads to self-discharge, reducing the actual capacity to bedelivered. In addition to these characteristics, some batteries, e.g.,NiCd batteries, are known to have memory effect, while Lithium-ionbatteries do not.

Apart from temporary changes in battery capacity noted above, batteriesmay lose their capacity to some extent due to unwanted side reactionsincluding electrolyte decomposition, active material dissolution, andpassive film formation, thereby increasing internal resistance andultimately causing a battery-cell failure. Several possible failuremodes exist, making the battery cell behavior difficult to predict.First, an open circuit can be a fail-safe mode for other battery cellsin the series chain including an open-circuited battery cell, becausethe open circuit limits further damage to the other battery cells.However, this failure mode may not be useful to the applications becauseall the battery cells in the series chain can become open-circuited andunusable. Second, a short circuit that has an abnormal low electricalresistance incurs almost no voltage drop, so that the rest of thebattery cells in the chain could be slightly overloaded while the wholebattery pack (i.e., a set of the battery cells) remains functional.Last, a possible explosion is avoided via a protection circuit thatdetects and stops an extremely high current.

FIG. 1 depicts an exemplary arrangement for a reconfigurable batterysystem 10. The reconfigurable battery system 10 is comprised generallyof a plurality of battery circuits 30 a-30 n arranged adjacent to oradjoined to each other. Each battery circuit 30-30 n has an associatedcontrol module 20 a-20 n. In an exemplary embodiment, the controlmodules 20 a-20 n are implemented by a controller 50 although it isenvisioned that the functions supported by the control modules 20 a-20 n(or portions thereof) may be partitioned amongst multiple controllers.As used herein, the term module may refer to, be part of, or include anapplication specific integrated circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and/or memory (shared,dedicated, or group) that execute one or more software or firmwareprograms, a combinational logic circuit, and/or other suitablecomponents that provide the described functionality. It is should beunderstood that software or firmware programs are implemented ascomputer executable instructions residing in a computer memory andexecuted by a computer processor.

The battery circuits 30 a-30 n are comprised of an input terminal 36 a,an output terminal 34 a, and a battery cell 32 a interposed between theinput terminal 36 a and the output terminal 34 a. The design of thedynamic reconfiguration framework is guided by a principle: one shouldbe able to bypass any battery cell. In addition, as few switches aspossible should be placed around a given battery cell to minimize costand improve reliability. In the exemplary embodiment, each batterycircuit further include four switches: an input switch 38 a (alsoreferred to as S_(I)) connected between the input terminal 36 a and thenegative terminal of the battery cell 32 a; a parallel switch 44 a (alsoreferred to as S_(P)) connected between the output terminal 34 a and thepositive terminal of the battery cell; a bypass switch 40 a (alsoreferred to as S_(B)) connected between the negative terminal of thebattery cell and a negative terminal of an adjacent battery circuit; anda series switch 42 a (also referred to as S_(S)) connected between thepositive terminal of the battery cell 32 a and the negative terminal ofthe adjacent battery circuit. Battery circuits 30 a-30 n areinterconnected by input terminal switches 46 a-46 n and output terminalswitches 48 a-48 n (also referred to as S_(IT) and S_(OT), respectively)which to allow the battery system to provide multiple terminals asfurther described below. While reference is made to a particular switcharrangement, other switch arrangements are within the broader aspects ofthis disclosure.

Control units 20 a-20 n configure the switches in the plurality ofbattery circuits to form different circuit arrangements. For example,battery cells may be configured in a series arrangement by setting theswitches in a given battery circuit 30 b as follows: input switch 38 bis set off; series switch 42 b is set on; bypass switch 40 b is set off;and parallel switch 44 b is set off, where on is a closed circuit acrossthe switch and off is an open circuit across the switch. When aplurality of cells are arranged in series, a cell 32 b can be bypassedby setting the switches as follows: input switch 38 b is set off; seriesswitch 42 b is set off; bypass switch 40 b is set on; and parallelswitch 44 b is set off. It is readily understood that switches inbattery circuits on either end of the series string may be configureddifferently to place the respectively cell in the series string or bebypassed.

To configure battery cells in parallel with each other, switches in agiven battery circuit 30 b are configured as follows: input switch 38 bis set on; series switch 42 b is set off; bypass switch 40 b is set off;and parallel switch 44 b is set on. Likewise, it is understood thatswitches in battery circuits on either end of the parallel grouping maybe configured differently to place the respectively cell in parallelwith the remaining cells. When a plurality of cells are arranged inparallel, a cell can be bypassed by setting all of the switches in thegiven battery circuit to off.

The architecture of the dynamic reconfigurable battery system 10 can berepresented as Ψ=(E,F,S,D), where E is an array of sensors,{E_(1, . . . ,)E_(i, . . . ,)E_(k)} each of which reads the voltage andthe current of a corresponding battery cell. F denotes an array offeedback switches, {F_(1, . . . ,)F_(i, . . . ,)F_(k)}, that thecontroller maintains to determine which cell to be bypassed. When abattery-cell failure in device i is detected, (F_(i), On) is turned. Sdenotes an array of the switches, {S_(1, . . . ,)S_(i, . . . ,)S_(k)},where S_(i) is composed of S_(i,I), S_(i,0), S_(i, B), S_(i, S),S_(i, P), S_(i, IT), and S_(i, OT). D is a set of battery devices,{D_(1, . . . ,)D_(i, . . . ,)D_(k)}. The connectivity of these devicesis thought of as an n_(s)×n_(p) matrix:

$\begin{matrix}\begin{pmatrix}D_{1,1} & \ldots & D_{1,n_{p}} \\\vdots & \ddots & \vdots \\D_{n,{n\; 1}} & \ldots & D_{n,n_{p}}\end{pmatrix} & (1)\end{matrix}$where n_(s) is the number of battery cells connected in a series chainand n_(p) is the number of the series chains connected in parallel. Theterms V_(d) and V_(a) denote the voltage demand and the average voltageof battery cells (or a set of battery packs), respectively. It isunderstood that the voltage demand is dictated by the application.Similarly, f_(N) is defined as:

$\begin{matrix}{f_{N} = {\sum\limits_{i = 1}^{k}{{}_{}^{}\left( F_{i} \right)_{}^{}}}} & (2)\end{matrix}$where (F_(i)) is an indication function, i.e., if (F_(i), Off) holds,then the function returns 1, else it returns 0.

During operation, the control unit for a given battery circuit monitorsan operational state of the battery cell in the battery circuit andcontrols switches in the battery circuit in accordance with theoperational state. In the exemplary embodiment, control unit 20 a-20 ncommunicates with two sensors 54 a-54 n and 56 a-56 n to monitor thebattery condition. For example, control unit 20 a-20 n monitors changesin the state of charge (SOC) and voltage of its battery cells viasensing devices 54 a-54 n and 56 a-56 n. The SOC of a battery cell maybe estimated by measuring and integrating the current flowing into andout of battery cell 32 a-32 n over time, called a Coulomb count. Inpractice, voltage and temperature may also be figured in as batteryvariables. Thus, function f_(V,T) (SOC, ∫Idt), that is based on thecontent of the coulomb count returns SOC. On the other hand, in general,direct voltage measurement is not accurate enough to be used as anindicator because of its dependency on the discharge rate andtemperature. Voltage may be estimated by applying a Kalman filter insome embodiments. Alternatively, one may assume that an integratedrecursive function, f_(V,I,T) (SOC, ∫Idt), is given and returns [V,SOC]. Other techniques for determining state of charge and/or voltage ofbattery cells are also within the scope of this disclosure. It isreadily understood that different types of sensors may be used tomonitor battery conditions.

At periodic monitoring intervals (Δt,), the controller 50 checks the SOCof each battery cell via the corresponding control unit 20 a-20 n andtriggers a rotation event if

$\begin{matrix}{\frac{\min\left( {{SOC}_{1},\ldots\mspace{11mu},{SOC}_{k}} \right)}{\max\left( {{SOC}_{1},\ldots\mspace{11mu},{SOC}_{k}} \right)} < \delta} & (3)\end{matrix}$holds, where δ denotes a threshold that bounds the maximum variation ofSOCs. The larger the δ, the more the battery cells become unbalanced.Furthermore, the variation needs to be adjusted with δ, in conjunctionwith Δt, because the larger the Δt, the larger the variation. Inparticular, Δt is inversely proportional to the discharge rate. Arotation event is an adjustment in the battery pack where health batterycells are rotated with other healthy battery cells for the purpose ofkeeping the cells healthy.

For discussion purposes, a faulty cell may be regarded as a battery cellthat can be charged as low as 80% nominal capacity and/or that hasvoltage as low as the cut-off voltage in a fully charged state. Thus,when battery cell i is determined faulty (F_(i), On) is turned incontrol unit i. Other criteria for determining a faulty cell are alsocontemplated.

At each monitoring interval (Δt,), the controller 50 also checks theaverage voltage and triggers a reconfiguration event unlessV _(d) ≦V _(a) *n _(s)

V _(d)+α  (4)holds, where α specifies an upper bounds of voltage unbalancing. It canbe observed that α is tuned based on the granularity in supply voltage.The reconfiguration event causes the controller to change the topologyof the battery circuits. A reconfiguration event typically occurs when abattery cell is determined to be faulty. A reconfiguration event mayalso occur when additional applications require a voltage supply,thereby requiring a multiple terminal configuration. Other types oftriggering reconfiguration events are also contemplated by thisdisclosure.

In the event of a battery cell failure or another triggeringreconfiguration event, a semantic bypassing mechanism configures batteryconnectivity. In general, the semantic bypassing mechanism implementspolicies for supplying a wide range of voltages while abiding by voltagebalancing across the parallel groups of the series chains. In anexemplary embodiment, two policies are implemented by the semanticbypassing mechanism although other policies are contemplated by thisdisclosure. The semantic bypass mechanism is implemented by thecontroller.

First, a constant-voltage-keeping policy is specified to keep the supplyvoltage as constant over the battery lifetime as possible in spite ofthe battery-cell failure. To this end, the series chain containing thefaulty battery cell is bypassed. However, it is possible that thevoltages of both used and unused healthy battery cells in the serieschain may drift apart over time, resulting in unbalanced voltagesbetween the battery cells within the series chain. For this reason, arotation event is triggered during the monitoring, reconfiguring thebattery-cell connectivity. For connectivity reconfiguration, batterycells at the lowest level of their SOC are singled out first.

FIG. 2 illustrates the reconfiguration of battery cells in a serieschain under the constant-voltage-keeping policy. In FIG. 2, twoconfigurations 60 a and 60 b are depicted. In a first configuration 60a, the last four battery cells are connected in series. In a secondconfiguration 60 b, controller 52 has connected the middle four batterycells in series in accordance with the constant voltage keeping policy.As can be seen, the healthy battery cells are being rotated, therebykeeping the voltage constant. As can be observed in the figure, thefaulty cell 62 is excluded in both configurations.

To implement a constant-voltage keeping policy, controller 52 mustdetermine how many battery cells should be bypassed. The number ofbattery cells to be bypassed is calculated as follows. Given V_(d),n_(s) is first calculated by

$\left\lceil \frac{V_{d}}{V_{a}} \right\rceil;$use of V_(a) offsets the nonlinear voltage drop during their lifetime.n_(p) is then derived from

$\left\lfloor \frac{f_{N}(\Psi)}{n_{s}} \right\rfloor,$where f_(N)(Ψ) indicates/returns the total number of battery cellsavailable to use. This equation leads to (f_(N)(Ψ)−n_(s)·n_(p)) healthybattery cells to be bypassed. This procedure repeats at periodicintervals (Δt) or upon initiation of a reconfiguration event.

Alternatively, a dynamic-voltage-allowing policy is defined to supportas many applications as required and to improve the maximum deliverablepower, given available battery cells, at the expense of a voltage dropthat corresponds to a single battery-cell voltage. Under thedynamic-voltage-allowing policy, one or more healthy battery cells in aseries chain may be singled out as shown in FIG. 3. To apply thispolicy, n_(p) remains fixed in accordance with the applicationrequirements and n_(s) is then calculated by

$\left\lfloor \frac{f_{N}(\Psi)}{n_{p}} \right\rfloor,$resulting in (f_(N)(Ψ)−n_(s)·n_(p)) healthy battery cells to bebypassed. As with the constant-voltage-keeping policy, the battery cellsare singled out based on their SOC. Likewise, this procedure repeats atperiodic monitoring intervals (Δt) or upon initiation of areconfiguration event.

These two policies may be further understood from the example, set forthbelow. Suppose three parallel groups, each of which has 4 battery cellsin series, the configuration is represented as

-   -   C1(O), C2(O), C3(O), C4(O)    -   C5(O), C6(O), C7(O), C8(O)    -   C9(O), C10(O), C11(O), C12(O)        where O indicates which of corresponding cells are being used.        Suppose that the voltage of each cell equals 1V so that each        series string outputs 4V. Assuming that C6 and C8 fail, two        cells in the other groups should rest in order to balance        voltages across the groups, resulting in this configuration:    -   C1(O), C2(O), C3(-), C4(-)    -   C5(O), C6(X), C7(O), C8(X)    -   C9(O), C10(O), C11(-), C12(-)        where X indicates the corresponding cells fail and - indicates        the cells rest.

The semantic bypassing mechanism will reconfigure battery connectivityin accordance with one of the two policies. If the constant-voltagekeeping policy is applied (i.e., demand voltage is 4V), then theresulting configuration is

C1(O), C2(O), C3(O), C4(O)

C5(O), C6(X), C7(O), C8(X), C9(O), C10(O)

C11(-), C12(-)

and the power output by this configuration is 4V*2 (=8P).

On the other hand, if the dynamic-voltage allowing policy is applied,then the resulting configuration is

-   -   C1(O), C2(O), C3(O),    -   C4(O), C5(O), C6(X), C7(O),    -   C8(X), C9(O), C10(O), C11(O), C12(-)        with the power output by this configuration being 3V*3 (=9P). In        the dynamic-voltage allowing policy, the number of parallel        groups (n_(p)) is not changed. Instead, by adjusting output        voltage with ns, the deliverable power may be increased.

These two policies are complementary to maximize the battery usability.In particular, according to the load demand which isapplication-specific, the constant-voltage keeping policy is appliedwhenever it is necessary. For example, a system that supportsdynamic-voltage scaling reduces down its system voltage when it has alow demand. To do this, the system often uses a step-down DC-DCconverter. A DC-DC converter may consume energy in the course ofconversion. In this case, instead of using the DC-DC converter, applyingthe constant-voltage keeping policy to the reconfigurable system allowsmore energy-savings. On the other hand, the dynamic voltage keepingpolicy is applied when multiple applications are to be accommodatedsimultaneously. Applications that require different voltage and powerneed a specific capacity, thereby defining in the number of parallelgroups, n_(p), needed and causing them to be fixed.

Once n_(s) and n_(p) are determined in accordance with the appliedpolicy, the controller 50 applies a connectivity configuration algorithmto achieve the desired circuit arrangement. An exemplary configurationalgorithm is set forth below.

Config (F, ns, np):  f ← find(F ≠1, F=TB+PB); /* Find cells available touse */  k ← f[1]; /* Select the first available cell */ for i ← 1 : np Signal (S_(k,l), On); /* Start a parallel-connected group */  while Fk= 1 /* Check whether the k-th bit in F is 1 */    Signal (S_(k,B), On);/* Bypass cellk */    k ← k +1;  end while  Signal (S_(k,S), On); /*Connect cells in series */  j ← 1;  while j < (ns − 1)    k ← k +1;  while Fk = 1    Signal (S_(k,B), On); /* Bypass cellk */    k ← k +1;  end while   Signal (S_(k,S), On); /* Connect cells in series */   j ←j +1;  end while  Signal (S_(k,P), On); /* End a parallel-connectedgroup */ end forThe connectivity configuration algorithm begins by determining whichcells in the system are available for use. When the k-th local controlunit reports a cell failure to the controller, the controller updatesits data structure, PB, to permanently bypass the faulty cell by settingPB(k) to 1, where PB is a bit-vector of size equal to the total numberof battery cells and k is the k-th bit of the bit-vector. The occurrenceof a cell failure requires a healthy cell in each parallel group tobecome inactive, i.e., temporarily bypassing it. This intentional bypassis tracked by the controller which maintains another data structure, TB,for temporary bypass, where TB is a bit-vector of size equal to that ofPB.

Starting with the first available cell, a parallel-connected group isconstructed by the algorithm. Each cell is evaluated sequentially.Healthy cells are connected in series; whereas, unhealthy or failedcells or healthy cells reflected by TB are bypassed. The procedurerepeats until n_(s) healthy cells are connected in series, therebyforming a parallel-connected group. The process moves to the nextparallel-connected group and repeats the procedure until n_(p)parallel-connected groups have been formed. It is envisioned that otherprocedures may be used to connect cells to achieve the desired circuitarrangement.

Unlike ideal cells, the output voltage of cells is not constant duringtheir discharge. That is, ns×Va drops nonlinearly, deviating from Vd.The deviation can be handled by using a DC-DC converter. A DC-DCconverter, however, dissipates energy in the form of generation of heat.The energy dissipation is gauged with conversion efficiency for theDC-DC converter (EFF_(DC-DC)) given byEFF_(DC-DC)=(I_(OUT)×V_(OUT))/(I_(N)×V_(IN)), where I_(IN) and V_(IN)each are input current and voltage to the DC-DC converter, and I_(OUT)and V_(OUT) are output current and voltage from it, respectively.EFF_(DC-DC) can be approximated to be between 75% and 95%, when theinput variation is not extreme; DC-DC converters are most efficient whenthe input voltage is closest to the output voltage. It is important tofind the right point in time for reconfiguration so as to minimize powerdissipation. Define a function of power: f_(DC-DC):V_(IN)×V_(OUT)→EFF_(DC-DC). Then, power dissipation is given byPD=(1−f_(DC-DC)(V_(IN), V_(OUT)))×V_(IN)×I_(IN). Once V_(IN) isdetermined, the constant-voltage keeping policy is applied

To minimize power dissipation, adjust V_(IN) to V_(d) by reconfiguringthe cell connectivity. Assuming the reconfiguration overhead is limitedby the switching overhead, the switching overhead includes the powerconsumption of transmitting a signal to a switch and turning on/off theswitch. This overhead in discrete-time seldom varies and is thusapproximated to be constant, resulting in a negligible amount of powerdissipation, compared to the energy dissipation in continuous time. Thecontroller self-configures the cell arrangement whenever a priori powerdissipation is greater than a posteriori power dissipation (after thereconfiguration) including the switching overhead, i.e., when satisfyingthe condition:(1−f _(DC-DC)(Vc,Vd))×PcΔtC>(1−f _(DC-DC)(V*d,Vd))×P*d×Δt _(C) −α×P*dwhere Vc denotes a priori terminal voltage, and V*d is an approximationof the demand voltage, Vd; and α is a switching overhead. With V*d, thecontroller recalculates n_(s) and n_(p). This criterion allows thecontroller to self-reconfigure the cell connectivity in real time.

The architecture described above may be extended to multiple batterypacks, where each battery pack is comprised of a reconfigurable batterysystem 10 as described above. In other words, each battery pack iscomprised of a plurality of battery circuits and a local controller thatcontrols the operation of the battery circuits. This extendedarchitecture further includes a global controller in data communicationwith each of the local controllers to coordinate functions amongst thebattery packs. Extension of the dynamic reconfiguration framework tomultiple battery packs is represented as γ=(E, F, S, Ψ), where Ψ={Ψ₁, .. . , Ψ₂, . . . , Ψ_(k)}. γ is configured by the global controller incooperation with the local controllers; whereas, each Ψ_(i) isconfigured via its respective local controller. The two policiesdescribed above can be implemented in and applied by the globalcontroller.

Global controller reconfigures the battery cells in γ in conjunctionwith local controllers, generating a wide range of supply voltages forthe load. Given V_(d), the global controller calculates the number ofbattery cells to be connected in series Ψ_(k) and γ, i.e.,

$\begin{matrix}{{{n_{s} \cdot N_{s}} \equiv \left\lceil \frac{V_{d}}{V_{a}} \right\rceil},} & (5)\end{matrix}$where n_(x)≦f_(N)(Ψ_(k)) is the number of battery cells in a serieschain in Ψ_(k) and N_(s)≦f_(N) (γ) is the number of battery cells in aseries chain in γ. This equation holds on the condition that iff_(N)(Ψ_(k))≦f_(N)(γ), then n_(s)≦N_(s) or if f_(N)(γ)≦f_(N)(Ψ_(k)),then N_(s)≦n_(s). After n_(s) and N_(s) are resolved subject to thecondition, n_(p) in Ψ_(k) is calculated by

$\left\lfloor \frac{f_{N}\left( \Psi_{k} \right)}{n_{s}} \right\rfloor.$Likewise, N_(p) (the number of series chains connected in parallel) in γis calculated by

$\left\lfloor \frac{f_{N}(\gamma)}{N_{s}} \right\rfloor.$As a consequence, the local controllers and the global controller applythe connectivity configuration algorithm with arguments of(Ψ,n_(s),n_(p)) and (Ψ, N_(s), N_(p)), respectively, thereby resultingin all the battery cells in and out of the battery packs configured intandem.

When a plurality of battery packs are arranged in parallel, it may benecessary to bypass one of the battery packs if the battery packincludes failed battery cells. Similar to the local controller, theglobal controller determines as failure when (F_(i), On) is detected.However, when Ψ_(i) is simply bypassed, some battery cells in Ψ_(i) maybecome unusable. To address this issue, the global controller performs apack-level bypassing decision algorithm. In this algorithm, the globalcontroller finds the minimum number of available cells across packs,denoted as n_(m), and then calculates how many cells will be bypassed ineach pack, based on previous values of n_(m). This decision issystematically made by the global controller via a decision function setforth below:

Y = {f_(N)(ψ₁),...,f_(N)(ψ_(k))}; B=0; Battery-Pack Bypassing Decision(Y, B):    n_(m)← min (Y);   /* the smallest number of cells acrosspacks */   n_(pb)← |{ ψ_(i) | n_(m)=f_(N)(ψ_(k))}|  /* the number ofpacks that have n_(m) */    n_(b)← Σ (Y− n_(m) );  /* sum of cells thatmay be bypassed */   if n_(b) ≧ (n_(m) × n_(pb) + B)     Battery-PackBypassing Decision     (Y − { ψ_(i) | for all k, n_(m)=f_(N)(ψ_(k))},n_(m)× n_(pb));   else     return n_(m);   end if

Two examples are provided below to better understand this decisionfunction. In a first example, suppose that there are 4 battery packseach of which initially has 6 cells. That is, [6, 6, 6, 6]. When 1, 2,and 2 cells have failed in packs 1, 2, and 3, respectively, (denoted by[5, 4, 4, 6]), we get n_(m)=4. Since the number of cells to be bypassedin each pack (denoted as n_(b)), i.e., the sum of (1, 0, 0, and 2) issmaller than n_(m)×2, the algorithm does not bypass packs 2 and 3.Instead, it decides to bypass 2 cells in pack 1 and 1 cell in pack 4. Inan second example, assume [4, 2, 3, 6]. In this example, n_(m)=2. Sincen_(b)=7 (i.e., 2+0+1+4) is greater than n_(m), pack 2 is bypassed,resulting in [4, 3, 6]. In this step, n_(m)=3. Since n_(b)=4 (i.e., 1+3)is smaller than n_(m)+n_(m)*(the previous n_(m)), the algorithm returnsn_(m) (i.e., the latest n_(m)). Each pack then bypasses its cells basedon n_(m). That is, 1, 2, 0, and 3 cells are bypassed in packs 1, 2, 3,and 4, respectively. In this way, the global controller can determinewhen and how to bypass a battery pack. When the local controller in eachpack receives the latest value of n_(m) from the global controller, eachlocal controller applies the constant-voltage keeping policy based onn_(m).

The reconfigurable framework described above can also be used to supportmultiple applications, where an application requires power from thebattery system. For example, in a vehicle, the starter motor, thewindshield wipers and the radio may all require power from the batterysystem. Each application defines an output voltage requirement V_(d) andmay be assigned a priority. The output voltage requirement V_(d) for anapplication k determines the number of cells in series N_(s,k) needed tomeet the requirement. The sum of N_(p,1), N_(p,2), . . . , N_(p,k),gives the total number of healthy battery cells. N(γ) leads to qparallel groups for all the applications.

Battery cells can then be allocated to each of the requestingapplications. If the number of battery cells needed to meet theapplication requirements exceeds the number of battery cells availablefor use, then the available battery cells are assigned to applicationsbased on the priority assigned to the applications. If the number ofbattery cells available for use exceeds the number of cells needed tomeet the application requirements, then the controller can allocateremaining cells. In either case, available battery cells are distributedfirst to high-priority applications, i.e., those with a high demandvoltage. This distribution continues until the remaining cells are notenough to be distributed. An exemplary allocation policy is defined asfollows.

Multi-Terminal-Based Grouping:

  /* {N_(s,1), . . . ,N_(s,i), . . . N_(s,k) | N_(s,1) > . . . >N_(s,i), > . . . > . . . N_(s,k)}*/  N_(p,1), . . . ,N_(p,i), . . .N_(p,k)]←1  $\left. q\leftarrow\left\lfloor \frac{N(\gamma)}{N_{s,1} + {\ldots\mspace{14mu} N_{s,i}} + {\ldots\mspace{14mu} N_{s,k}}} \right\rfloor \right.;$ for j← 0;   ${{if}\mspace{14mu}\left( {{{fN}(\gamma)} - {q{\sum\limits_{i = 1}^{k}\; N_{s,i}}}} \right)} < {\sum\limits_{I = 1}^{J}\;{N_{p,i} \cdot N_{s,i}}}$   N_(p,j) ← 0;   end if  end for  [N_(p,1), . . . ,N_(p,i), . . .N_(p,k)]← [N_(p,1), . . . ,N_(p,i), . . . N_(p,k)]+q;  return [N_(p,1),. . . ,N_(p,i), . . . N_(p,k)]Thus, the controller allocates the power source of N_(s,k)×N_(p,k) toeach application k. In the case of the extended reconfigurableframework, the allocation policy is implemented by the globalcontroller.

Once the battery cells have been allocated, the controller(s) configurethe battery system accordingly. The battery system is first configuredto provide an input terminal and an output terminal for each applicationhaving allocated battery cells. To do so, input-terminal switches 46a-46 n and output-terminal switches 48 a-48 n are controlled to providemultiple terminals. For instance, when input-terminal switches (S_(i,l),On) and output-terminal switches (S_(i,P), On) stay for all batterycircuits 30 a-30 n, the interface for the battery pack has a singleinput terminal and a single output terminal. Conversely, to segment thebattery circuits 30 a-30 n and provide multiple terminals, selectinput-terminal switches and output-terminal switches can be set to Off.For each application segment, the battery system is then configured tomeet the application requirements using the semantic-bypassing mechanismdescribed above.

Large-scale battery cells, e.g., for EVs and HEVs, are packed in such away that n_(s) battery cells are connected in series, providing therequired supply voltage, and n_(p) parallel groups are connected inparallel, determining flows of the current (I), resulting in therequired capacity. The capacity, because of the nonlinearities ofbatteries, cannot be derived simply by the ideal battery capacityequation:C≡T·1,  (6)where T is the discharge time (the battery lifetime). Instead, empiricalPeukert's relation models nonlinearities for the case of a constantcurrent load by introducing an empirical parameter as: C≡T·1^(α), whereα>1 is called Peukert's value, which typically ranges between 1.2 and1.4.

For purposes of the reconfigurable battery management system, thenonlinearity may be modeled using discretization of a flow of thecurrent. That is, real-world systems are characterized by loads that arevariable over time. Such variable loads may be approximated bypiece-wise constant loads, represented by a set of M current levels (i₁,. . . , i_(M)), in which M is used to characterize the load and isdetermined by the quantization interval, Δt(≡t_(i)−t_(i+1)) which is afraction of the total operation time, T. That is

${{I_{i}(t)} \equiv {\sum\limits_{i \equiv 1}^{M}{{i_{i} \cdot 1_{\lbrack{t_{i - 1},t_{i}}\rbrack}}(t)}}},$where 1_(A)(t) is an indicator function. So, the smaller the Δt, thehigher the accuracy in the characterization of the load. In the casewhere Δt≡T, the load is constant. The patterns of the load can beobtained via empirical measurements, resulting in a discharge profilefor a battery cell or a pack of battery cells. Thus, the model of Eq.(6) generalizes toC=T·I _(i)(t).  (7)

The total load is the sum of the current that is loaded from individualparallel groups, i.e., I=I₁+ . . . +1_(i)+ . . . +I_(np) and it isuniformly distributed at some point in time within a certain acceptablethreshold in discrepancy, leading to I=n_(p)·1_(i). This results in

$\begin{matrix}{C = {T \cdot {\frac{I(t)}{n_{p}}.}}} & (8)\end{matrix}$

When cell failures occur, the number of available parallel groups equalsn_(p)−N(t), where N(t) is the total number of failures occurred in thebattery-cell array by time t. In the ABS, the number of availableparallel groups is defined as:

$\begin{matrix}{{n_{A} \equiv {n_{p} - \left\lceil \frac{N(t)}{n_{s}} \right\rceil}},{0 \leq {N(t)} \leq {n_{p} \cdot n_{s}}}} & (9)\end{matrix}$

Since the numbers of these failures that occur in disjoint timeintervals are independent, N(t) is Poisson distributed with abattery-cell failure rate, λ. So, the average total number of cellfailures that occur by time t is proportional to t, resulting in λ·t.This equation is applied to Eq. (9) yielding:

$\begin{matrix}{{C = {{T \cdot \frac{I(t)}{n_{A}}} \equiv {T \cdot \frac{I(t)}{n_{p} - \left\lceil \frac{\lambda - t}{n_{s}} \right\rceil}}}},} & (10)\end{matrix}$where

$t < \frac{n_{p} \cdot n_{s}}{\lambda} \leq {T.}$

On the other hand, in a legacy scheme, the load for a series chain ofoperational battery cells increases in proportion to the total number ofcell failures across the n_(p) parallel groups as:

$\begin{matrix}{n_{L} = \left\{ \begin{matrix}{{n_{p} - {N(t)}},} & {{0 < {N(t)}},{< n_{p}}} \\{0,} & {n_{p} < {N(t)} \leq {n_{p} \cdot n_{s}}}\end{matrix} \right.} & (11)\end{matrix}$

The linear increase in the load is due to the fact that it fails toreuse any healthy battery cells in the series chain containing a faultycell. So, the available capacity in following the legacy scheme iscalculated by

$\begin{matrix}{{C \equiv {T \cdot \frac{I(t)}{n_{L}}} \equiv {T \cdot \frac{I(t)}{n_{p} - {\lambda \cdot t}}}},} & (12)\end{matrix}$where

$t < \frac{n_{p}}{\lambda} \leq {T.}$

Therefore, the higher λ, the more lifetime gain over the legacy scheme;it is also inversely proportional to the number of battery cells inseries, n_(s).

Either of the two policies described above is applied, based on theconfiguration of the battery-cell connectivity. To maximize battery-cellutilization the capacity of power that the entire battery cells deliveris selected as a criterion to compare the two policies. If an m×n matrixrepresents a combination of n_(s) battery cells in a series chain andthere are n_(p) parallel groups, any element of battery cells in thematrix is assumed to become faulty independently of others. Forinstance, when one battery cell fails, (n_(s)−1) ·n_(p) of power isprovided, based on the dynamic-voltage allowing policy. For simplicity,it is assumed the element of each battery cell is capable of 1 volt and1 ampere, while n_(s)·(n_(p)−1) of power based on the constant-voltagekeeping policy. So, the breakeven point in selecting the policy is foundwhen n_(s)=·n_(p). When more than one battery cell fails, the number ofbattery cells left unused due to the faulty-cell detouring reflects ameasure of the capacity. In other words, a ratio (r) of the number ofcolumns (c) to the number of rows (w) counted on faulty cells in thematrix can be a factor in the decision to make, comparing with the totalsize of the matrix. So, the breakeven point is determined by

$\begin{matrix}{{r \equiv \frac{c}{w} \equiv \frac{n_{p}}{n_{s}}},} & (13)\end{matrix}$and hence, when

${r > \frac{n_{p}}{n_{s}}},$the dynamic-voltage allowing policy is chosen, providing more capacityof power than the constant-voltage keeping policy.

An evaluation methodology is first described and then the performance ofthe described architecture is evaluated in comparison with a legacyscheme that cannot configure the battery-cell connectivity online. Themetrics used for evaluation of battery performance include the batterylifetime and the supply voltage. The lifetime is proportional to thetotal capacity of the battery cells/packs, while the supply voltagedetermines the deliverable power. The battery dynamics were simulatedusing Dualfoil, which is widely used for designing multiple batterysystems. For a more detailed explanation of Dualfoil, reference is madeto “Modeling of galvanostatic charge and discharge of thelithium/polymer/insertion cell” J. of Power Sources, 140(6)1526-1533,2003. Using Dualfoil is sufficient to demonstrate the way the batteryconnectivity is dynamically reconfigured.

The reconfiguration framework effectively “masks” the effects of abattery-cell failure, thus extending the battery lifetime, while thelegacy scheme significantly suffers battery-capacity loss and hencereduces the lifetime. The battery lifetime is computed with the maximumdeliverable power and the amount of current constantly drawn from thebattery pack. Obviously, the more the battery-cell failures, the higherthe reduction in the battery lifetime. FIG. 4 illustrates the results ofthe comparison of the battery lifetimes. Clearly, the legacy schemeloses a significant amount of span as the number of faulty battery cellsincreases. The reason for this is that the failure of one battery cellresults in the loss of the series chain including the faulty batterycell. By contrast, the reconfiguration framework reuses the remaininghealthy battery cells in the series chain as backup cells. So, despiteadditional battery-cell failure in other chains, they are replaced withsurviving healthy battery cells. FIG. 4 shows the fault-tolerancecapability of the proposed reconfiguration framework. For instance, whenλ·t≡6 through 9 and λ·t≡12 through 15, the battery-pack's lifetimeremains constant irrespective of an increase in number of battery-cellfailures. The difference in lifetime between the two mechanisms getslarger as the frequency of battery-cell failures gets higher. As can beseen in FIG. 5, the lifetime gain achieved by the reconfigurationframework grows substantially with an increase in number of batterycells in a series chain (n_(S)) in each parallel group, thus enhancingthe availability of backup battery cells. This is effective even for thecase of connecting two battery cells in series (i.e., n_(s)=2),achieving a factor of 5 gain. Clearly, the more the battery cells inseries, the larger the gain.

The dynamic-voltage-allowing policy aims to meet the demand ofwide-ranging supply voltages from different applications while keepingdeliverable power maximum. FIG. 6 illustrates changes in the demandvoltage and the corresponding maximum deliverable power resulting from a25-battery-cell pack that is based on the configuration of settingactual supply voltage and capacity of each battery cell to 3.6 Volts and1.3 AH, respectively, with jitter of 2.5% allowed. So, maximumdeliverable power is bounded by between an estimated 114 W and 120 W.This power can be delivered in a combination of 5 parallel groups and 5battery cells of a series chain in each group (i.e., (5, 5)), or oneparallel group with 25 battery cells in series (i.e., 25, 1)).Interestingly, a good range of supply voltages, corresponding to thegroup circled in FIG. 6, is provided while keeping maximum deliverablepower reasonably constant. This implies that appropriately turning thebattery connection can improve the utilization of battery cells whilemeeting the demand of the underlying applications. In the meantime, theconnectivity of (9, 2) or (13, 1) appears inefficient with respect tothe utilization of battery cells. However, failure of any battery cellor a voltage drop can be resolved by virtually replacing them withbackup battery cells, thereby maintaining the required voltage level.

The dynamic-voltage-allowing and constant-voltage-keeping policies aredevised for different purposes: the former aims to meet the demand ofwide-ranging supply voltages, while the latter is to sustain anacceptable range of supply voltage against battery failures or apossible voltage drop during the battery lifetime, both with thedeliverable power kept maximum. So the two policies can be compared withrespect to the deliverable power. FIG. 7 shows the distribution of powermagnitudes between the constant-voltage-keeping and thedynamic-voltage-allowing policies. In battery connectivity, whenn_(s)>n_(p), the dynamic-voltage-allowing policy is effective insupplying the maximum deliverable power, while when n_(p)>n_(s), theconstant-voltage-keeping policy is a better choice. The reason for thislies in the utilization of unused battery cells/packs. Obviously, thebreak-even point occurs when n_(s)=n_(p).

As mentioned earlier, since the voltage drop is unavoidable, theconstant-voltage-keeping policy is applied to keep the supply voltageabove or equal to the demand voltage while the supply voltage is beingmonitored. The monitoring interval (Δt) is directly associated with adegree to which the system may suffer due to the voltage drop below thedemand. The higher the frequency of monitoring, the shorter the time anapplication suffers, but the higher the overhead of monitoring. FIG. 8Ashows changes in supply voltage with two different discharge ratesduring the lifetime of a 700-battery-cell pack. It is assumed that eachbattery cell is discharged independently, following the distribution ofdischarging a Lithium-ion battery that is simulated with theconfiguration of providing output voltage of 4.3 volts and nominalcapacity of 1.3 AH. Demand voltage (V_(d)) for an application is assumedto be 600 volts. In the case where the battery pack is discharged at Crate, in FIG. 8B, when the battery pack is monitored every Δt (=10), itis detected at the 10^(-th) time interval when the supply voltage dropsbelow V_(d), reconfiguring the battery pack connectivity into 4 parallelgroups with 143 battery cells in a series chain, i.e., (143, 4),providing an estimated 604 volts. In the case of C2 rate, in FIG. 8C,the underlying application suffers 5 times more battery-capacity lossthan at the normal discharge rate. In particular, the more steeply doesthe supply voltage drop, the larger the difference between the supplyand demand voltages. This case can be improved by reducing themonitoring interval (Δt=10). As can be seen in FIG. 8D, with themonitoring interval halved (Δt=5), on-time detection of the voltage dropis improved by 67%.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

What is claimed is:
 1. A reconfigurable battery system comprising: aplurality of battery circuits interconnected to each other, each circuithaving: a negative input terminal; a positive output terminal; a batterycell with a positive terminal and a negative terminal interposed betweenthe input terminal and the output terminal; an input switch connectedbetween the input terminal and the negative terminal of the batterycell; a parallel switch connected between the output terminal and thepositive terminal of the battery cell; a bypass switch connected betweenthe negative terminal of the battery cell and a negative terminal of anadjacent battery circuit; and a series switch connected between thepositive terminal of the battery cell and the negative terminal of theadjacent battery circuit; and a control unit that receives an outputcriteria and, in accordance with the output criteria, controls the inputswitch, the parallel switch, the bypass switch and the series switch inthe plurality of battery circuits to output a voltage to at least twopositive output terminals while substantially maximizing power deliveredby the circuit arrangement, where the output criteria defines a numberof positive output terminals at which to output a voltage and a voltagerequirement for each output.
 2. The reconfigurable battery system ofclaim 1 wherein the control unit determines a number of battery cellsavailable for use from the battery cells in the plurality of the batterycircuits and allocates the battery cells available for use to eachoutput defined by the output criteria.
 3. The reconfigurable batterysystem of claim 2 wherein the control unit receives a number of parallelcell groupings and configures the input switch, the parallel switch, thebypass switch and the series switch in the plurality of battery circuitsto form a circuit arrangement that minimizes the number of battery cellsavailable for use that are bypassed in the circuit arrangement.
 4. Thereconfigurable battery system of claim 2 wherein the control unitdetermines a number of cells to comprise each parallel cell grouping bydividing the number of battery cells available for use by the number ofparallel cell groupings, where the cells comprising a parallel cellgrouping are arranged in series with each other.
 5. The reconfigurablebattery system of claim 1 wherein the control unit configures the inputswitch, the parallel switch, the bypass switch and the series switch inthe plurality of battery circuits by determining a number of batterycells arranged in series are needed to meet the voltage outputrequirement and determining a number of battery cell groups which can bearranged in parallel to each other from the number of battery cellsavailable for use, where each battery cell group is comprised of astring of battery cells arranged in series that meet the voltage outputrequirement.
 6. The reconfigurable battery system of claim 5 wherein thecontrol unit configures the input switch, the parallel switch, thebypass switch and the series switch in the plurality of battery circuitsto form a circuit arrangement which outputs a voltage that meets thevoltage output requirement when the number of battery cell groupsexceeds the number of battery cells arranged in series that meet thevoltage output requirement.
 7. A reconfigurable battery systemcomprising: a plurality of battery circuits adjoined to each other, eachbattery circuit having: a negative input terminal; a positive outputterminal; a battery cell with a positive terminal and a negativeterminal interposed between the intput terminal and the output terminal;a plurality of switches interconnecting the battery cell with a batterycell in an adjacent circuit and configurable to place the battery cellin series with the battery cell in the adjacent battery cell, or placethe battery cell in parallel with the battery cell in the adjacentbattery circuit, or disconnect the battery cell from the battery cell inthe adjacent circuit; and a control unit that receives an outputcriteria and, in accordance with the output criteria, controls theswitches in each of the battery circuits to output a voltage to at leasttwo positive output terminals while substantially maximizing powerdelivered by the circuit arrangement, where the output criteria definesa number of positive output terminals at which to output a voltage and avoltage requirement for each output.
 8. The reconfigurable batterysystem of claim 7 wherein the control unit determines a number ofbattery cells available for use from the battery cells in the pluralityof the battery circuits and allocates the battery cells available foruse to each output defined by the output criteria.
 9. The reconfigurablebattery system of claim 8 wherein the control unit receives the voltageoutput requirement for a given output and configures the switches in theplurality of battery circuits to form a circuit arrangement whichoutputs a voltage that meets the voltage output requirement for thegiven output.
 10. The reconfigurable battery system of claim 9 whereinthe control unit that determines a number of battery cells allocated tothe given output, determines a number of battery cells arranged inseries are needed to meet the voltage output requirement and determinesa number of battery cell groups which can be arranged in parallel toeach other from the number of battery cells allocated to the givenoutput, where each battery cell group is comprised of a string ofbattery cells arranged in series that meet the voltage outputrequirement.
 11. The reconfigurable battery system of claim 10 whereinthe control unit configures the switches in the plurality of batterycircuits to form a circuit arrangement which outputs a voltage thatmeets the voltage output requirement when the number of battery cellgroups exceeds the number of battery cells arranged in series that meetthe voltage output requirement.
 12. The reconfigurable battery system ofclaim 9 wherein the control unit receives a number of parallel cellgroupings for a given output and configures the switches in theplurality of battery circuits to form a circuit arrangement thatminimizes the number of battery cells available for use that arebypassed in the circuit arrangement.
 13. The reconfigurable batterysystem of claim 12 wherein the control unit determines a number of cellsto comprise each parallel cell grouping by dividing the number ofbattery cells allocated to the given output by the number of parallelcell groupings, where the cells comprising a parallel cell grouping arearranged in series with each other.
 14. The reconfigurable batterysystem of claim 13 wherein the control unit configures the switches inthe plurality of battery circuits to form a circuit arrangement thatminimizes the number of battery cells available for use when the numberof cells to comprise each parallel cell grouping exceeds the number ofparallel cell groupings.
 15. The reconfigurable battery system of claim7 wherein the plurality of switches are further defined as an inputswitch connected between the input terminal and the negative terminal ofthe battery cell; a parallel switch connected between the outputterminal and the positive terminal of the battery cell; a bypass switchconnected between the negative terminal of the battery cell and anegative terminal of an adjacent battery circuit; and a series switchconnected between the positive terminal of the battery cell and thenegative terminal of the adjacent battery circuit.
 16. A reconfigurablebattery system comprising: a plurality of battery circuitsinterconnected to each other, each circuit having: a negative inputterminal; a positive output terminal; a battery cell with a positiveterminal and a negative terminal interposed between the input terminaland the output terminal; an input switch connected between the inputterminal and the negative terminal of the battery cell; a parallelswitch connected between the output terminal and the positive terminalof the battery cell; a bypass switch connected between the negativeterminal of the battery cell and a negative terminal of an adjacentbattery circuit; and a series switch connected between the positiveterminal of the battery cell and the negative terminal of the adjacentbattery circuit; an input-terminal switch interposed between the inputterminal and an input terminal of the adjacent battery circuit; anoutput-terminal switch interposed between the output terminal and anoutput terminal of the adjacent battery circuit; and a control unit thatreceives an output criteria and, in accordance with the output criteria,controls the input switch, the parallel switch, the bypass switch andthe series switch in the plurality of battery circuits to output avoltage to at least two positive output terminals while substantiallymaximizing power delivered by the circuit arrangement, where the outputcriteria defines a number of positive output terminals at which tooutput a voltage and a voltage requirement for each output.
 17. Thereconfigurable battery system of claim 16 wherein the control unitdetermines a number of battery cells available for use from the batterycells in the plurality of the battery circuits.
 18. The reconfigurablebattery system of claim 16 wherein the control unit receives a voltageoutput requirement for the battery system and configures the inputswitch, the parallel switch, the bypass switch and the series switch inthe plurality of battery circuits to form a circuit arrangement whichoutputs a voltage that meets the voltage output requirement.